Media Guardian: Word of mouse
This article provides a good discussion of the key elements of viral marketing, giving insight into how the process works from a marketer's perspective.
The story covers a lot of ground, kicking off with a classic example of a campaign in action, where an unknown band gained serious exposure by spoofing a threat of litigation from Metallica. The spoof claim was that they'd ripped off Metallica chords: "We're not saying we own those two chords, individually - that would be ridiculous. We're just saying that in that specific order, people have grown to associate E, F with our music."
